1. Field of the Invention
This invention relates to a semiconductor pressure sensor having strain gauges provided on a diaphragm portion for detecting pressure applied thereto.
2. Description of the Related Art
A conventional semiconductor pressure sensor includes a sensor chip 10 shown in FIG. 1 having a thin diaphragm portion 10a indicated by a dotted line in the figure. The diaphragm portion 10a has four piezo-resistance members (strain gauges) 1a to 1d made of an impurity diffusion layer thereon, two of which are center gauges 1a, 1b arranged at the central portion thereof and the other two of which are side gauges 1c, 1d arranged at the periphery thereof. The gauges 1a to 1d are electrically connected to form a bridge circuit as shown in FIG. 2 so that an electrical signal is output from terminals O.sub.A, O.sub.B in accordance with pressure. To form this constitution, an aluminum (Al) wiring member is provided around the diaphragm portion 10a to be electrically connected to the strain gauges 1a to 1d, and the sensor output is taken out from a circuit portion not shown to which the wiring member 2 is connected.
As shown in FIG. 3, the sensor chip 10 is fixed to a glass base 11 by anode-bonding, and the glass base 11 is adhered to an inside wall of a casing 13 made of polybutylene terephthalate (PBT) or the like by silicone system adhesive 12. Silicone gel 14 is coated on the surface of the sensor chip 10. In this constitution, the casing 13 can produce thermal distortion that adversely and innegligibly affects the sensor chip 10. Therefore, as shown in FIG. 3, a plate 15 made of 42 alloy is adhered to the lower surface of the glass base 11 by silicone system adhesive 16 to reduce the adverse effect of the thermal distortion of the casing 13 to the sensor chip 10. However, it is found that the sensor output still varies as a result of thermal hysteresis.